Method for lowering noise and providing offset correction in a transimpedance amplifier

ABSTRACT

A system for receiving signals (e.g., optical signals) includes an input device, an amplification device, and a feedback device. The amplification device receives a signal from the input device and includes a transimpedance portion. The transimpedance portion includes a first section having a plurality of elements (e.g., resistors and transistors) and a second section having a plurality of elements (e.g., resistors and transistors). One or more of the elements (e.g. transistors or resistors) in the first and second sections are mismatched to introduce a systematic offset in the transimpedance stage, to make the net input referred offset of the amplification device unidirectional. The feedback device (e.g. an integrator) is coupled to an output of the amplification device and an input of the transimpedance portion to provide a unidirectional offset correction to the amplification device for reduced noise enhancement.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application is a continuation if U.S. Ser. No. 10/227,257, filedAug. 26, 2002 (now U.S. Pat. No. 6,882,218 B2), which is incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to using offset correction to lowernoise in a transimpedance stage of an amplifier.

2. Background Art

Transimpedance amplifiers (TIAs) and limiting amplifiers (LAs) aretypically used in optical receivers, and other devices. The arepositioned at front end stages of a system to convert current from adetector (e.g., a photodetector) to voltage output, which is then fed toa clock and data recovery (CDR) circuit. TIAs in high speedcommunications (e.g., 10 GHz/s baud rates) are often faced withstringent requirements for reducing the input referred noise. This isneeded to maximize the signal-to-noise ratio (SNR) of the TIAs. Thisresults in lower bit error rates (BERs) when the TIAs are used inoptical receiving devices (e.g., photodetectors, optical sensors,optical detectors, or the like). Often, the limiting amplifier (LA)receives a signal from the TIA to limit a signal generated by the TIAbefore the signal is sent into the clock and data recovery circuit(CDR). In other cases, TIA's themselves can be implemented as atransimpedance first stage followed by one or more amplifier stages.Usually, a feedback device is positioned between an output of the TIAand an input of the transimpedance first stage of the TIA to generate afeedback signal that removes unidirectional currents D.C. offset or anyother D.C. offset that exists in all the stages of the amplifier.Unfortunately, the feedback device results in additional noise in thedesign of the transimpedance amplifier.

Therefore, what is needed is an improved topology for a transimpedancefirst stage, which can reduce noise in the TLA to an acceptable level.What is also needed other mechanisms to reduce noise specifically in thefirst stage of the transimpedance stage, which contribute a significantpart of the overall noise.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a system including an input device (e.g.,a photodetector), an amplification device, and a feedback device. Theamplification device receives a signal from the input device andincludes a transimpedance portion that converts the input current to avoltage. The transimpedance portion includes a first section having aplurality of elements (e.g., resistors and transistors) and a secondsection having a plurality of elements (e.g., resistors andtransistors). The elements (e.g., transistors and resistors) in thefirst and second sections are mismatched to introduce a systematicoffset in the transimpedance stage to make a net input offset at theamplification device unidirectional. The feedback device (e.g., anintegrator) can be coupled to an output of the amplification device andan input of the transimpedance portion to provide a single endedunidirectional offset correction to the amplification device for reducednoise enhancement.

The present invention further provides a transimpedance stage of anamplifier. The transimpedance stage includes a first section having aplurality of elements (e.g., resistors and transistors) and a secondsection having a plurality of elements (e.g., resistors andtransistors). The transistors in the first and second sections aremismatched, and when implemented as MOSFETs, have different width tolength ratios.

The present invention also provides a method including the step ofproviding at least a first element in a first section of a stage of anamplification system. The method also includes the step of providing atleast a first, similar element in a second section of the stage of theamplification system. The method also includes the step of introducing asystematic offset to make a net offset unidirectional by having thefirst element in the first section have a different impedance value thanthe first similar element in the second section.

Further embodiments, features, and advantages of the present inventions,as well as the structure and operation of the various embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1 shows a system with a transimpedance amplifier according toembodiments of the present invention.

FIG. 2 shows a transimpedance first stage in the system of FIG. 1.

FIG. 3 shows a feedback device in the system of FIG. 1.

FIG. 4 shows a transimpedance first stage coupled to a feedback devicein the system of FIG. 1 according to other embodiments of the presentinvention.

The present invention will now be described with reference to theaccompanying drawings. In the drawings, like reference numbers indicateidentical or functionally similar elements. Additionally, the left-mostdigit(s) of a reference number identifies the drawing in which thereference number first appears.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a system (e.g., an optical receiving system) 100 accordingto embodiments of the present invention. System 100 includes an opticalreceiving device (e.g., photodetector, optical detector, or opticalsensor) 102 that feeds a signal (e.g., a current) into a transimpedanceamplifier (TIA) 104, which converts the signal to a voltage signal andamplifies the signal for later processing. In some embodiments, thesignal is a current based on an optical signal detected by opticalreceiving device 102. TIA 104 feeds back an output signal through afeedback device (e.g., a D.C. loop circuit or an integrator) 106 to aninput of the TIA 104. TIA 104 can include a first stage (e.g., atransimpedance stage) 108, first through n second stages (e.g., gainstages) 110-1 to 110-n, and a third stage (e.g., a driver device orstage) 112. There can also be a converter (e.g., a voltage to current(V/I) converter) 114 positioned between feedback device 106 and TIA 104.

In some embodiments, a large gain can be required through TIA 104. Thiscan cause amplification of the offset produced through a first few gainstages 110. This can be especially true when using complimentary metaloxide semiconductor (CMOS) devices, where the offsets from theindividual first few gain stages 110 can be large even after carefullayout matching. Thus, a servo mechanism (e.g., feedback device 106) canbe used to cancel out the input referred offset of the entire system100.

The D.C. offsets of the individual gain stages 110 can be positive ornegative. Photodetector 102 produces unidirectional positive offsetsbecause it can only force current in one direction. Thus, a positive ornegative combined offset will be present at the output of the TIA 104after summing of the gain stage offsets and the photodetector offsets.The feedback device 106 usually cancels out the summed offsets.

FIG. 2 shows a circuit diagram of a transimpedance first stage 108.MOSFETs or any other type of transistors can be used without departingfrom the scope of the present invention. Transimpedance first stage 108includes a first section 200 and a second section 202. First section 200includes a transistor 204 (e.g., a MOSFET) coupled between Vdd andground. First section 200 also includes a first resistor (e.g., afeedback resistor) 206 connected between a gate and drain of transistor204 and a second resistor (e.g., a load resistor) 208 coupled betweenVdd and the drain of MOSFET 204. A node Out P is at a connection offirst and second resistors 206 and 208, respectively, and the drain ofMOSFET 204. Second section 202 has similar connections for MOSFET 210,resistors 212 and 214, and node Out N.

With continuing reference to FIG. 2, first section 200 receives thesignal from photodetector 102 and uses MOSFET 204 as an amplifier withresistor 206 in feedback to achieve transimpedance, while maintaininghigh bandwidth. Second section 202, which as discussed above is areplica of first section 200, provides a replica bias voltage Out N tothe subsequent gain stage 110-1. A single ended signal is converted to adifferential signal at gain stage 110-1.

FIG. 3 shows the feedback device 106 according to an embodiment of thepresent invention. Feedback device 106 can be configured as anintegrator including an operational amplifier 300 having a resistor 302at its input and a capacitor 304 coupled across it. Offset signals canbe at very low frequencies (e.g., a D.C. signal), so integrator 106 overtime can build up a finite output voltage corresponding toinfinitesimally small input signals. An ideal integrator during stableoperation has an output voltage tending to provide a “correctionoffset,” so that its input is equal to zero. In practice, however,integrator 106 may not have an infinite gain at D.C.; it will produce aninput voltage that is close to, but not equal to, zero.

Again with reference to FIGS. 1 and 2, and with continuing reference toFIG. 3, because the input to TIA 104 is current, when an integrator isused as feedback device 106, then converter 114 is placed after feedbackdevice 106. As shown in FIG. 2, converter 114 can include transistors(e.g., MOSFETs) 216 and 218. MOSFET 216 can convert voltage Vp fromfeedback device 106 to Ip, which is input to transimpedance stage 108 atnode IN. Similarly, MOSFET 216 can convert voltage Vn from feedbackdevice 106 to In, which is input to transimpedance stage 108 at nodeINN. Also, because the implementation of gain stages 110 aredifferential for better noise immunity and power supply rejection,integrator 106 can have differential inputs and outputs (e.g., outputsthat form intop and inton). Thus, transistors 204 and 210 provides a“correctional” offset current at the inputs IN and INN that isdifferential in nature, which can provide both positive and negativeoffset correction.

The embodiment shown in FIGS. 1–3 has some drawbacks. Although secondsection 202 is configured to provide just D.C. bias voltage to thesubsequent stage 110-1 (where differential conversion happens), thefeedback resistor 212 contributes an equal amount of input referrednoise. This is also true of MOSFET 210 and resistor 214, but their noisecontribution can either be the same or smaller than that attributed toresistor 212. Also, MOSFETS 216 and 218 contribute noise, even thoughtheir purpose is to cancel D.C. offset current. The noise becomes worseas the current through MOSFETs 216 and 218 increases. MOSFETs 216 and218 can carry current (e.g., I-216 and I-218, which are not shown in thefigures) that only contribute I-216–I-218 to provide cancellation withtotal input referred offset current. However, based on the common modeoutput of amplifier 300 in integrator 106, the common mode currentthrough MOSFETS 216 and 218 can be large.

For example, to correct 50 μA input offset current, MOSFETs 216 and 218can be forced by feedback device 106 to have 650 μA and 700 μA ofcurrent, respectively. To correct a 1.2 mA input offset current, MOSFETs216 and 218 can have 2.2 mA and 1 mA of current, respectively. Thegreater the bias current through MOSFETs 216 and 218, the more will betheir contribution to the input referred noise current. Essentially,this large common mode current as a result of differential offsetcancellation leads to extra noise. On the other hand, because bothpositive and negative current may need to be corrected, either adifferential circuit or a push-pull stage having the same problem wouldbe needed. Using single ended, single device feedback may provide only aunidirectional cancellation, which is unacceptable.

FIG. 4 shows transimpedance stage 400 according to another embodiment ofthe present invention that overcomes the above mentioned drawbacks. Theconfiguration of transimpedance stage 400 is similar to transimpedancestage 108 with two sections 402 and 404 having transistors 406 and 408and load resistors 410 and 412. However, only first section 402 has afeedback resistor 414, as discussed in more detail below. Thisconfiguration eliminates the problem of bi-directional offset byintroducing a known offset in transimpedance stage 400. This offset isintroduced utilizing either transistors 406 and 408 or load resistors410 and 412. An offset is introduced by mismatching impedance values ordevice sizes, so that equivalent input referred offset by thisunidirectional systematic offset is enough to overcome the effect of theother random offsets in transimpedance stage 400 and the first few gainstages 110.

With continuing reference to FIG. 4, either transistors 406 and 408 orresistors 410 and 412 are mismatched by a predetermined amount. Forexample, a width-to-length (W/L) ratio of transistor 406 can be made apredetermined amount larger (e.g., maybe a range of 10–20%) than a W/Lratio of transistor 408. To ensure that the sum of this systematicoffset and the random offsets that could exist is in the same directionas the photodetector input offset, that is all offset correction thatnow needs to be done is unidirectional. By making first section 402nonsymmetrical with respect to second section 404, noise can be furtherreduced by eliminating feedback resistor 212 that was in second section202, which can contribute a large amount of noise. In essence,transistor 408 becomes diode connected.

Thus, as shown in FIG. 4, by eliminating a feedback resistor in secondsection 404, a substantial noise reduction is achieved (e.g., a 25–30%reduction in noise). Also, by using single ended feedback, no commonmode current is needed, which: (1) eliminates the noise of an additionaldevice, and (2) reduces the required current to cancel normal amounts(e.g., 1.2 mA) of net input referred offset current. This allows a sizeor impedance of transistor 408 to be reduced, reducing both currentthrough and noise from transistor 408. These two factors combine toresult in a reduction of input referred noise due to transimpedancefirst stage 400 by up to 30–40%. This is a great advantage in systemsrequiring high speeds and low noise.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A method, comprising: (a) forming a differential amplifier in atransimpedance section of an amplification device; (b) coupling a firstsection of the differential amplifier, the first section having aplurality of elements including at least a transistor, to a secondsection of the differential amplifier, the second section having aplurality of elements including at least a transistor; (c) mismatchingone or more of the plurality of elements in the first section withrespect to corresponding ones of one or more plurality of elements inthe second section, such that a systematic offset in the transimpedancestage is introduced, which makes a net offset of the amplificationdevice unidirectional; and (d) coupling a feedback device to an outputof the amplification device and an input of the differential amplifierof the transimpedance portion to provide a unidirectional offsetcorrection to the amplification device.
 2. The method of claim 1,wherein the one or more plurality of elements in the first section andthe second section includes at least a resistive element and step (c)comprises: mismatching an impedance value of the resistive element inthe first section with respect to an impedance value of the resistiveelement in the second section.
 3. The method of claim 2, wherein theresistive element in the first section has a higher impedance value thanthe resistive element in the second section.
 4. The method of claim 2,wherein the resistive element in the first section has a lower impedancevalue than the resistive element in the second section.
 5. The method ofclaim 1, wherein step (b) comprises: using field effect transistor (FET)devices as the transistors.
 6. The method of claim 5, wherein a FET inthe at least one FETs in the first section has a larger width to lengthratio than a FET in the at least one FETS in the second section.
 7. Themethod of claim 1, wherein step (b) comprises only providing the firstsection with a transimpedance resistor.
 8. The method of claim 1,wherein step (d) comprises using an integrator as the feedback device.9. The method of claim 1, further comprising: (e) coupling a converterbetween the feedback device and the amplification device.
 10. The methodof claim 9, wherein step (e) comprises using a voltage-to-currentconverter as the converter.
 11. The method of claim 1, wherein step (c)comprises using similar elements for the one or more of the elements inthe second section being mismatched with respect to the elements in thefirst section.